CEE is an abbreviation of Certification of Electrotechnical Equipment. Standards were published by the IECEE (International commission on rules for the approval of electrical equipment).
Full name of CEE publication 7 is "Specification for plugs and socket-outlets for domestic and similar uses". CEE 7, dating back to the early 1950s, unifies standards utilized in several continental European countries.
Beside CEE 7 two other CEE plug/socket standards have existed; a potential source of confusion.

CEE type	Main characteristics
7/1	16A unearthed socket that accepts each of the CEE 7 type plugs. Most socket have a shallow recess. Sockets that accept plugs with a ca. 38 mm round base go back to the early twentieth century. In a growing number of European countries unearthed sockets are no longer allowed in newly build houses.
7/2	Round base 16A unearthed plug that fits in CEE 7/1 sockets only. CEE 7/2 plugs will gradually disappear in favour of more versatile 16A CEE 7/17 or 2.5A CEE 7/16.
7/3	16A earthed socket with two peripheral earth contacts. The design, also known as Schuko, dates back to the late 1920s (see origin).
7/4	16A plug with earth clips, often indicated as Schuko plug. Designed for CEE 7/3 sockets. Plugs fit also in CEE 7/1 sockets, but without earth connection.
7/5	16A socket with an earth pin, known as French type, which is in fact an incorrect name. The socket type was introduced in France in the 1950s, but already two decades earlier in Belgium (see origin).
7/6	16A plug that matches with socket CEE 7/5. Fits also in CEE 7/1 socket, but without earth connection.
7/7	CEE 7/4 do not fit in CEE 7/5, and CEE 7/6 do not fit in CEE 7/3 sockets. Plug type 7/7 has been designed to solve this incompatibility problem. Cords fixed to irons, vacuum cleaners etc.have nearly always fixed CEE 7/7 plugs, but rewirable plugs are also available.
7/16	2.5A not earthed plug that is compatible with each of the three CEE sockets and also with Danish, Italian and Swiss standard domestic sockets. A true (continental) Europlug. Plugs have pin with a diameter of 4.0 mm and must have insulating sleeves.
7/17	Unearthed plug rated at 16A and is compatible with each of the CEE 7 and Danish sockets. Most CEE 7/17 plugs are molded to appliance cords, but in a couple of countries rewirable plugs are available. CEE 7/17 have 4.8 mm pins without sleeves.